1. Field of the Invention
This invention relates to methods and devices for stabilizing a diode laser optical output with respect to temperature variations. This invention further relates to methods and devices for monolithic integration of a diode laser with a driver circuitry.
2. Description of Related Art
A semiconductor diode laser emits an optical output power in response to an electrical input signal. The input signal, e.g. an injection current, is applied to the laser contacts being provided by an external control circuitry, also known as laser driver circuitry. It is well recognized that co-location of electronic circuits and light-emitting devices, e.g. a driver circuitry and a diode laser, is frequently the most cost-effective solution due to the higher level of integration. However, it is difficult to integrate monolithically the diode laser and the driver circuitry. For example, an integration of III-V optoelectronic components with CMOS-based (or similar) silicon electronics is a complicated problem because of the difference in lattice parameters and thermal expansion coefficients. Integration of III-V optoelectronic components with III-V electronic components seems to be more realistic owing to similarity in materials used. Moreover, III-V Field Effect Transistors (FETs), in particular GaAs MEtal-Semiconductor FETs (MESFETs), GaAs High-Electron Mobility Transistors (HEMTs) and Pseudomorphic HEMTs (PHEMTs), and InP HEMTs and PHEMTs, are among the most efficient high-speed microelectronic devices currently available. In this context the use of III-V electronics can be advantageous because high-speed modulation of a diode laser would require corresponding high-speed operation of a driver circuitry.
However, FETs usually need to be formed on a semi-insulating substrate, while the semiconductor laser usually needs a conductive III-V substrate or buffer. Therefore, formation of FET-based driver circuitry on top of the diode laser layers is impossible because FETs operating at high-frequencies are short-circuited by parallel conductivity of these conductive layers. The driver circuitry and the diode laser, however, can be located beside each other. To do so, the conductive layers of the diode laser structure should be locally etched out and the layers of the driver circuitry should be located at the recess. Alternatively, the layers of driver circuitry should be locally etched out and the layers of the laser structure should be located at the recess. The most challenging part of this approach is filling the recess. One technique proposed was Epitaxy-on-Electronics (EoE), in which the desired layered structure is grown directly in the recesses. One example of implementation of this method is presented in A. C. Grot, et al., “Integration of LED's and GaAs circuits by MBE regrowth”, IEEE Photonics Technology Letters, Vol. 6, No. 7, July 1994, pp. 819-821. Another technique, Aligned Pillar Bonding (APB), involves growing the desired heterostructures in an inverse sequence on a separate wafer, etching it into a pattern of pillars, mirroring the pattern of recesses, and then aligning and bonding the two wafers together. EoE and APB techniques are discussed in U.S. Pat. No. 6,888,178 and in E. Atmaca, et al., “Development of RM3 technology to integrate P-i-N photodiodes on Si-CMOS for optical clock distribution”, The International Conference on Compound Semiconductor Manufacturing Technology “Sharing Ideas Throughout the Industry”. Both of these techniques are often difficult to realize taking into account a large depth of diode laser structures, especially vertical cavity surface emitting lasers.
Thus, methods for the monolithic integration of the FET-based high-speed driver circuitry and the diode laser are not well developed in the prior art.
It is strongly desired, in particular for optical communication systems, to have a stable optical output signal from a diode laser regardless of the temperature variation. It would be very advantageous to achieve temperature insensitivity of the internal laser parameters such as the threshold current and the slope efficiency. However, in spite of certain progress in this direction, the temperature sensitivity of the laser parameters in the current state of the art is typically unsatisfactory. Also, it is quite difficult to achieve a stabilization of the laser temperature during a long period of laser operation. This is due to two factors. First, the ambient temperature may change rapidly and unpredictably in a wide range. Second, the temperature of the active region of the laser affects the laser parameters. This temperature can be at least twenty degrees different than the temperature of the heat-sink. Therefore, a diode laser needs temperature compensation in order to produce stable optical output. To this end, driver circuitry should vary an input signal which is supplied to the diode laser. If the input signal represents a combination of the bias signal and the modulation signal, both components should be varied independently.
One solution known in the art is optical feedback. A photodetector actually senses the intensity of the laser radiation emitted from the back facet of the laser, which is in known proportion with the laser output power emitted from the front facet. The output of the photodetector is used to increase the drive current to compensate for diminished output with rising temperature. However, use of backface monitoring does not completely solve the problem of laser output changes as a function of temperature. In particular, this technique is incapable of adjusting the optical modulation amplitude. Another known solution for temperature compensation of the laser output is an adjustment of the drive signal in response to the temperature variation. In particular, U.S. Pat. No. 5,043,992 discloses a laser driver which includes a reference circuit mounted in thermal contact with the laser. The reference circuit produces a current component that is proportional to the absolute temperature. This provides for the required increase in the modulation current to compensate for temperature variations in the laser output. The driver circuitry and the laser may be constructed on the same integrated circuit.
Thus, there is a need for a diode laser with temperature compensation monolithically integrated with high-speed driver circuitry.